In recent times, earth-abundant, and inexpensive iron-catalysis has become more fascinating and a viable system for environmentally benign reactions. The economical, greener and ample supply of iron salts, coupled with their lack of toxicity, makes them ideal candidates for both the academic and industrial applications. However, search for an efficient catalytic system based on iron-catalysts for sustainable catalysis is extremely rare due to propensity of iron complexes to participate in one electron chemistry as opposed to traditional two electron transformation ubiquitous in the precious second- and third-row transition-metals and to the challenge of working with paramagnetic materials (mostly kinetically labile) and underdeveloped mechanistic studies. Design of a new catalytic system based on earth-abundant, and inexpensive metal catalysts for fundamentally important organic transformations under environmentally-benign conditions is an important paradigm in chemical synthesis.
Iron as a catalyst in chemical synthesis has been experienced renaissance due to not only its abundance and occurrence in various biological systems as essential key element but its activity and selectivity in the synthetic transformations are edge over the state-of-art of precious metal catalysts in certain case. Many iron based catalysed have been proved to be efficient and promising homogeneous catalytic systems by inventing the appropriate non-innocent ligand systems which interfere actively in the catalytic process. Graphene oxide (GO) sheets are emerging as a new class of carbocatalysts. Conventionally, graphite is oxidized to graphene oxide and exfoliated into submicrometer-sized, water-dispersible flakes to produce graphene oxide sheets. The presence of oxygen functional groups on the aromatic scaffold of GO allows these sheets to mediate ionic and nonionic interactions with a wide range of molecules.
Article titled “Efficient and Highly Selective Iron-Catalyzed Reduction of Nitroarenes” by R V Jagadeesh et al. published in Chem. Commun., 2011,47, 10972-10974 reports pyrolysis of iron—phenanthroline complexes supported on carbon leads to highly selective catalysts for the reduction of structurally diverse nitroarenes to anilines in 90-99% yields.
Article titled “Nitrogen-Doped Graphene and Its Iron-Based Composite As Efficient Electrocatalysts for Oxygen Reduction Reaction” by K Parvez et al. published in ACS Nano, 2012, 6 (11), pp 9541-9550 reports a cost-effective synthesis of NG by using cyanamide as a nitrogen source and graphene oxide as a precursor, which led to high and controllable nitrogen contents (4.0% to 12.0%) after pyrolysis. NG thermally treated at 900° C. shows a stable methanol crossover effect, high current density, and durability when catalyzing ORR in alkaline solution. Further, iron (Fe) nanoparticles could be incorporated into NG with the aid of Fe (III) chloride in the synthetic process. This allows one to examine the influence of non-noble metals on the electrocatalytic performance.
Article titled “Graphene-supported iron-based nanoparticles encapsulated in nitrogen-doped carbon as a synergistic catalyst for hydrogen evolution and oxygen reduction reactions” by Wang et al. published in Faraday Discuss., 2014,176, 135-151 reports graphene-supported iron-based nanoparticles encapsulated in a nitrogen-doped carbon (Fe@N—C) hybrid material acts as an efficient HER and ORR catalyst. The hybrid material showed higher electrocatalytic activities than graphene sheets or Fe@N—C alone, which is probably attributed to the synergetic role of nitrogen-doped graphene and Fe@N—C towards the electrocatalytic reactions.
Article titled “Non-precious metal nanoparticles supported on nitrogen-doped graphene as a promising catalyst for oxygen reduction reaction: Synthesis, characterization and electrocatalytic performance” by H Ghanbarlou et al. published in Journal of Power Sources, 2015, 273 , pp 981-989 reports nitrogen-doped graphene (NG) based non-precious metal catalysts is used as a catalyst for oxygen reduction reaction (ORR). Nanoflower-like NG with designed nitrogen types is directly synthesized using a low temperature solvothermal process and then Fe, Co and Fe—Co nanoparticles are precipitated onto the NG using a modified polyol method.
Article titled “Nitrogen-Doped Graphene-Activated Iron-Oxide-Based Nanocatalysts for Selective Transfer Hydrogenation of Nitroarenes” by RV Jagadeesh et al. published in ACS Catal., 2015, 5 (3), pp 1526-1529 reports Nanoscaled iron oxides on carbon modified with nitrogen-doped graphene (NGr) as an excellent catalysts for the chemoselective transfer hydrogenation of nitroarenes to anilines.
Article titled “Nitrogen-self-doped graphene-based non-precious metal catalyst with superior performance to Pt/C catalyst toward oxygen reduction reaction” by C He et al. published in J. Mater. Chem. A, 2014,2, 3231-3236 reports A new, simple and scalable synthesis methodology is invented for an N-self-doped graphene-based non-precious Fe catalyst (Fe—N-graphene) for the oxygen reduction reaction (ORR) both in acidic and alkaline media. The electrochemical characterization shows that this Fe—N-graphene catalyst possesses outstanding electrocatalytic ORR activity.
Article titled “Fe—N—C oxygen reduction catalysts supported on vertically aligned carbon nanotubes” by G C K Liu et 1. published in Applied Catalysis A: General, 2008, 347(1), pp 43-49 reports Non-noble metal electrocatalyst (Fe—N—C) for the oxygen reduction reaction (ORR) sputter deposited onto films of vertically aligned carbon nanotubes (VACNT) and tested by the rotating ring disk electrode (RRDE) technique.
Recently, it has been recognized that iron-based dehydrogenation process with liberation of hydrogen gas as the by-product, competes with the state-of-art of precious metal based catalytic systems. Indeed it has a significant development in the conversion of renewable feedstocks into value-added products with complete atom-efficiency.
Dehydrogenation of organic compounds that allow chemical integration of abundant organic substrates (e.g. alkanes and alcohols) into valuable products and find manifold applications in chemical synthesis as well as in energy storage systems. Traditionally, the dehydrogenation of organic compounds has been achieved with the aid of stoichiometric amount of sacrificial oxidants and/or hydrogen acceptors. Recent development of smart catalytic systems allows oxidant-free and acceptorless conditions with the liberation of dihydrogen, an energy carrier. This strategy has been further extended to dehydrogenative cross-couplings and cascade annulations to enable a range of structurally diverse molecules at single operation.
Article titled “Reversible catalytic dehydrogenation of alcohols for energy storage” by P J Bonitatibus et al. published in PNAS, 2015, 112 (6), pp 1687-1692 reports reversible acceptorless dehydrogenation of secondary alcohols and diols on iron pincer complexes and reversible oxidative dehydrogenation of primary alcohols/reduction of aldehydes with separate transfer of protons and electrons on iridium complexes.
Article titled “Fe-catalyzed acceptorless dehydrogenation of secondary benzylic alcohols” by H Song et al. published in ACS Catal., 2014, 4 (9), pp 2889-2895 reports an operationally simple, economical, and environmentally benign acceptorless Fe-catalyzed dehydrogenation of various secondary benzylic alcohols to afford the corresponding ketones and H2. A simple in situ mixture of readily available Fe(III) acetylacetonate, 1,10-phenanthroline, and K2CO3 was identified as an active catalyst for this transformation.
Article titled “Hydrogenation using iron oxide-based nanocatalysts for the synthesis of amines” by R V Jagadeesh et al. published in Nat Protoc., 2015; 10 (4); 548-57 reports the preparation of nanoscale iron oxide-based materials and their use in the catalysis of different hydrogenation reactions. Pyrolysis of a Fe(OAc)2-phenanthroline complex on carbon at 800° C. under argon atmosphere results in the formation of nanoscale Fe2O3 particles surrounded by nitrogen-doped graphene layers.
Article titled “Well-Defined iron catalysts for the acceptorless reversible dehydrogenation-hydrogenation of alcohols and ketones” by S Chakraborty et al. published in ACS Catal., 2014, 4 (11), pp 3994-4003 reports acceptorless dehydrogenation of alcohols accomplished with well-defined and inexpensive iron-based catalysts supported by a cooperating PNP pincer ligand. Benzylic and aliphatic secondary alcohols were dehydrogenated to the corresponding ketones in good isolated yields upon release of dihydrogen. Primary alcohols were dehydrogenated to esters and lactones, respectively. Mixed primary/secondary diols were oxidized at the secondary alcohol moiety with good chemoselectivity.
Among the various fundamental organic reactions which have industrial application, one such reaction is direct conversion of alcohols to value added products such as carbonyl compounds. Dehydrogenation of alcohols with homogeneous catalysts based on precious metals such as Ru, Jr, and Rh are reported for aerobic oxidation. Metal catalyzed oxidation permit the use of mild, inexpensive, and environmentally benign oxidizing agents, such as O2 or H2O2. Other mild oxidizing agents used as hydrogen acceptors include included ketones, olefins and amine-N-oxides.
However, in terms of atom economy and the use of hydrogen as fuel, oxidant-free reaction to give carbonyl products via acceptor less alcohol dehydrogenation is desirable.
Catalytic acceptorless dehydrogenation of primary alcohols to esters by soluble Fe(II)-PNP pincer complex is reported in Science, 2013, 341, 249.
Traditional approaches to imine synthesis mainly involve condensation of an aldehyde or a ketone with a primary amine, and oxidative condensation of primary amines using strong oxidant. Catalytic oxidative condensation of primary amines using less-toxic oxidants such as oxygen, hydrogen peroxide is well explored. Kobayashi et al. in Chem. Rev. 2011, 111, 2626-2704 reports that imines and their derivatives constitute diverse class of organic compounds and have been mostly identified by their profound application in organic synthesis, pharmaceuticals, and agricultural chemicals.
Article titled “Synthesis and Characterization of Iron-Nitrogen-Doped Graphene/Core-Shell Catalysts: Efficient Oxidative Dehydrogenation of N-Heterocycles” by X Cui et al. published in J. Am. Chem. Soc., 2015 July 31, 137 (33), pp 10652-10658 reports synthesis and characterization of iron oxides surrounded by nitrogen-doped-graphene shells immobilized on carbon support (labeled FeOx@NGr—C). Active catalytic materials are obtained in a simple, scalable and two-step method via pyrolysis of iron acetate and phenanthroline and subsequent selective leaching. The optimized FeOx@NGr—C catalyst showed high activity in oxidative dehydrogenations of several N-heterocycles.
However, the reported methods use highly reactive aldehydes, costly metal and high pressure. Also, the water formed during the reported methods leads to back reaction which affects the yield of the desired product. Therefore it is the need to develop an eco-benign and atom-economical process for making value-added imines and/or N-heterocyclic compounds in high yield. Accordingly, the present inventors developed an inexpensive, environment friendly, benign magnetically separable catalyst using earth-abundant elements such as carbon and iron which can be used efficiently in oxidant-free catalytic dehydrogenation of alcohols and amines to corresponding value added product with hydrogen as the only by product.